The LISA interferometer: impact of stray light on the phase of the heterodyne signal

The Laser Interferometer Space Antenna is a foreseen gravitational wave detector, which aims to detect 10−20 strains in the frequency range from 0.1 mHz to 0.1 Hz. It is a triangular constellation, with equal sides of m, of three spacecraft, where heterodyne interferometry measures the spacecraft distances. The stray light from the powerful transmitted beam can overlap with the received one and interfere with the heterodyne signal. We investigated the contribution of random phase variations of the stray photons to the noise of the heterodyne signal. A balanced detection scheme more effectively mitigates this adverse effect than a separation of the frequencies of the transmitted and local radiation. In the balanced scheme, in order to limit the phase noise to picometer level, the incoherent power of the stray light must be kept below about 10 nW W−1 for an asymmetry of the recombination beam splitter of 1%.